The present invention relates to a new and improved construction of strand guide assembly -- also known in the art as a roller apron or roller apron assembly -- for supporting a partially solidified cast strand in a continuous casting installation, such roller apron comprising a number of pairs of rollers of different diameter successively arranged in the direction of travel of the strand, each of the rollers being supported at two bearing or support locations. As a matter of convenience in the disclosure of the invention to follow the strand guide assembly will be conveniently simply referred to as a roller apron or roller apron assembly.
The guide rollers of continuous casting installations or plants are subjected to a high mechanical alternating load during each revolution of such guide rollers owing to the ferrostatic pressure acting upon the skin or shell of the continuously cast strand. In the case of slab casting installations such high mechanical loads can exceed 100 tons per roller. Apart from the foregoing mechanical load the rollers are subjected to an additional load owing to the shock-like temperature increase of the roller surface each time that it comes into contact with the hot strand. These mechanical and thermal loads, depending upon the strength of the material from which the rollers are fabricated, require a predetermined diameter of the rollers. Since the ferrostatic pressure decreases as a function of the distance from the molten metal level in the continuous casting mold, prior art roller aprons are constructed such that in sections, viewed in the discharge direction of the continuously cast strand, there are provided rollers possessing increasing diameters.
The momentarily employed roller diameter dictates a predetermined minimum spacing between the rollers and thus also the size of the unsupported surface of the strand between the rollers. The mutual dependency between the permissible roller load, the strand width, the thickness of the strand shell or skin, the diameter of the rollers and the spacing of successive rollers limits, in the case of large strand formats or shapes, the casting speed since there must be prevented deleterious bulging-out of the cast strand and metal breakouts. Hence, strands having a width of, for instance, 2.5 to 3 meters cannot be rationally cast with the heretofore known apparatuses.
There are already known to the art roller aprons wherein the strand guiding or guide rollers are constructed of a number of parts and supported over the width of the strand in a number of locations by bearings or supports. In this way, it is possible to maintain relatively small both the diameter and thus also the mutual spacing of successive guide rollers. However, a drawback of such complicated construction resides in the fact that for each multi-part constructed strand guide roller there are required three or more bearings. Significant in this regard is the fact that the bearings are not readily accessible for repair or replacement work, constituting a significant drawback. Additionally, in the case of driven rollers, the individual roller sections must be interconnected by couplings. Hence, these complicated roller aprons or strand guides do not fulfill the requirements concerning a simple, sturdy and operationally reliable construction.
Continuing, there is also already known in this particular field of technology a withdrawal and straightening machine for continuous casting installations wherein the rollers which are particularly loaded by the straightening forces are dimensioned to be larger in size than the normal or standard rollers by virtue of their more intensive loading. In order to insure for a uniform bearing spacing the rollers which immediately neighbor the larger size rollers are constructed to be of smaller size than the standard rollers. With this prior art construction either the aforementioned smaller size rollers are under-dimensioned or the standard rollers which are loaded with the same intensity are over-dimensioned. With this proposal there is not attained any general reduction in the unsupported surfaces of the cast strand and thus an increase of the casting speed.